The present invention relates generally to satellites, and more particularly, to methods that use satellite state vector prediction to provide three-axis satellite attitude control.
Future satellites built by the assignee of the present invention may be tasked to perform electric orbit raising in order to reduce bipropellant requirements to reduce system mass (cost) and/or increase mission life. The Telstar 8 satellite deployed by the assignee of the present invention is tasked to perform on-orbit stationkeeping operations using electric propulsion.
The assignee of the present invention has developed a basic concept of operations for electric orbit raising in which a main (bipropellant) satellite thruster quickly raises future electric thruster-equipped (SPT-equipped) satellites from their initial transfer orbits to intermediate orbits above the Earth's radiation belts, where solar array degradation is no longer a problem. From this point, electric thrusters (SPTs) are used to complete orbit raising to the geosynchronous station over a relatively longer time frame (several weeks to months).
The electric thrusters perform this portion of the orbit raising more efficiently than a bipropellant thruster could. This is due primarily to the greatly increased specific impulse (Isp) of the electric thruster. The electric thruster has a much lower thrust level than its bipropellant counterpart, and, as a result, must operate over long periods of time and large portions of the orbit. Once on orbit, the electric thrusters switch roles to perform daily stationkeeping maneuvers to maintain the satellite in it's stationkeeping box.
The electric orbit raising concept developed by the assignee of the present invention is described in great detail in U.S. patent application Ser. No. 09/328,805, filed Jun. 9, 1999, entitled "Practical Orbit Raising System and Method for Geosynchronous Satellites", which is assigned to the assignee of the present invention. For the purposes of this description, the term "transfer orbit" may be the initial separation geosynchronous transfer orbit or any subsequent orbit prior to reaching the final geosynchronous orbit.
In order to accomplish the orbit-raising task, two major problems must be addressed. First, solutions must be developed to define the "correct" thrust profile that will inject the satellite from its new transfer orbit to its final orbit. The term "correct" means the desired thrust profile. This may be a profile optimized to do one or more of the following: 1) minimize orbit raising propellant, 2) minimize time to complete orbit raising, 3) maximize on-orbit life, 4) maintain some required power level on board during electric thruster (SPT) operations, or 5) minimize some cost function that is some combination of the above and/or any other variable that one might contrive. Once the desired orbit-raising thrust-vector trajectory has been defined, the second problem that must be addressed is controlling the satellite attitude along the resulting trajectory to achieve the goals of the strategy while maximizing power and meeting sensor and telemetry and control (T&C) constraints.
The present invention focuses on the task of controlling the attitude during the electric thruster (SPT) phase of orbit raising, "flying" the profile, in a manner that is operationally feasible. While it is not a formal system requirement, as the method is described, it will become apparent that the use of an on-board orbit propagator such as that used on Sirius Radio satellites developed by the assignee of the present invention, with some modifications, will allow a practical real time control solution with minimal ground system reliance and/or commanding. Therefore, an on-board orbit propagator may be a derived requirement in such a system.
Therefore, it is an objective of the present invention to provide for improved methods that use satellite state vector prediction to provide three-axis satellite attitude control.